This invention relates generally to field devices for process measurement and control. Specifically, the invention concerns a field device with integrated temperature control, in order to provide more direct protection against low-temperature extremes.
Field devices cover a broad range of process management devices that measure and control fluid parameters such as pressure, temperature and flow rate. Field devices have broad utility in a wide variety of applications including manufacturing, hydrocarbon processing, bulk fluid handling, food and beverage preparation, water and air distribution, environmental control, and precision chemical, glue, resin, thin film and thermoplastic applications.
Field devices include transmitters, which are configured to measure or sense process parameters, and controllers, which are configured to modify or control such parameters. Transmitters comprise sensor modules that sense fluid parameters, such as pressure transducers that generate analog voltage or current signals that characterize a process pressure. Sensor modules also include temperature sensors, flow sensors, PH sensors, level sensors, and a variety of other sensor devices for sensing or characterizing other process variables and fluid parameters.
In contrast to transmitters, controllers utilize control modules to modify or influence a process parameter, rather than simply characterize it. Control modules typically produce control outputs that represent target values for the parameter, such as analog current outputs used to position valves or otherwise achieve a desired flow rate. Control modules also include temperature controllers, pressure regulators, level controllers, and other process control devices.
More generalized field devices include pressure/temperature transmitters and other multi-sensor transmitters, as well as integrated flow controllers with both sensor and control functionality. Additional field devices combine sensing and control functions, such as hydrostatic tank gauge systems that simultaneously measure and regulate a number of related pressures, temperatures, fluid levels and flow rates.
Field devices are often exposed to a wide range of environmental effects, including temperature extremes due to changing ambient conditions or sunlight exposure, and process-related effects such as high temperature fluids or cryogenic flow. Low temperature extremes in particular can degrade transmitter and controller response, and produce offset, drift, or signal noise in associated electronics such as A/D (analog-to-digital) or D/A (digital-to-analog) converters. Extremely low temperatures can even result in malfunction or failure of the field device.
Environmental control is therefore a significant consideration for field device design. In particular, environmental enclosures are commonly utilized to shade field devices from sunlight and other radiant energy sources, and heaters are often added to protect from low temperature extremes. Existing heating and heat control technologies are unfortunately somewhat indirect, and suffer from increased power consumption and a larger overall size envelope. This raises costs and reduces installation flexibility, particularly in remote or limited-access applications. There is thus a continuing need for improved environmental control techniques that provide more direct temperature control, with a reduced impact on installed size and overall efficiency of the field device.